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SQLite was originally coded in such a way that the answer to all questions in the chart below would be "Yes".
But the experiments run on other SQL engines showed that none of them worked this way.
One of the key goals when building applications is to make sure the data you enter into the database meets all your business rules.
Data validation is a critical part of your application to ensure your data meets the requirements developed by your business analysts.
Codd also introduced the use of the lowercase Greek omega (ω) symbol to represent Null in database theory.
In SQL, is a reserved word used to identify this marker. A null value indicates a lack of a value - a lack of a value is not the same thing as a value of zero in the same way that a lack of an answer is not the same thing as an answer of "no".
This involved making NULLs indistinct for the purposes of the SELECT DISTINCT statement and for the UNION operator in a SELECT. This seems somewhat arbitrary, but the desire to be compatible with other engines outweighed that objection.Question: I am trying to create a table that will disallow a NULL value from being inserted into my table, based upon a condition, using an if-then-else.I know that I can easily disallow NULL values on insert with the NOT NULL clause, but I want the value to change depending on the value of another inserted column: In sum, I want to create this complex business rule as a conditional check constraint, using if-then-else logic to check for complex Boolean expressions?You can have multiple check constraint for a single column.If data being inserted or updated violates a check constraint the database engine will not allow the INSERT or UPDATE operation to occur.
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Null (or NULL) is a special marker used in Structured Query Language (SQL) to indicate that a data value does not exist in the database.